Organic Light Emitting Diode (OLED) display technology is a new wave in flat panel displays and expected to challenge the most popular Liquid Crystal Displays (LCDs) in many applications. A basic structure of OLED consists of an anode and a cathode between which is sandwiched a multi-layer organic thin film that generates light when electrons and holes recombine, as a result of sufficiently applied voltage, at the organic layer. The organic films and at least one metallic film are sensitive to the presence of water vapor and oxygen. As a result, one of the major problems faced by OLED is its longevity. The presence of oxygen and water vapor beyond one part per million (ppm) inside the device can deteriorate the performance of OLED. The material undergoes oxidation in the presence of oxygen and water vapor. Metallic film employed for cathode layer is sensitive for de-lamination under this environment. The end result is the marked decrease in brightness and formation and spreading of ‘black spots’ that is characterized by islands of no light emission. There are two mechanisms of degradation of light emission. One is due to the intrinsic nature of the organic materials that is responsible for overall uniform decrease in light emission and the other is due to the reaction of oxygen and water vapor that results in ‘black spots’. Uniform degradation of light output, due to intrinsic aging nature of organic materials, can still render the device useful as a display with reduced brightness, whereas the black spots tend to spread, both during operation and storage, and thus render the device useless as a display. Hence the latter mechanism is more serious than the first.
Foregoing demands serious consideration in the hermetic sealing of OLED devices against oxygen and water vapor, particularly the materials employed for hermetic sealing medium. The seal process is normally carried out in a moisture and oxygen controlled, less than 1 ppm, glove box, after the OLED has been processed and automatically transferred to the glove box. A mathematical model for the permeation of moisture through seals is given by,t=(V.L/P.A.R.T)In[(P0−P1)/(P0−P2)]Where t is the time in seconds to reach P2                 V is the free volume of the device (cm3)        L is the diffusion path length or seal width (cm)        P is the permeability of the sealant (g/cm sec torr)        A is area of the seal exposed to the permeant (cm2)        R is the gas constant (3465 torr cm30/K gH2O)        T is the temperature (0K)        P0 is the external water vapor pressure in torr        P1 is the initial internal water vapor pressure in torr        P2 is the final internal water vapor pressure in torr        
Most important factors to be considered for a good hermetic sealing is the geometry of the seal namely, width, length and height and the material of the seal. Width of the seal, for planar geometry like OLED, plays a major role due to the permeation length and hence dimensions of width in the range of microns is totally insufficient. It needs to be in fractions of millimeter. Other significant factor is the material of the seal. FIG. 1, to follow in the description, gives permeation rates of moisture in various materials like, silicones, epoxies, fluorocarbons, glasses and metals for different thickness (width) of the seal. It can be seen from the figure, the best material of choice is the metal. This invention relates to a metal seal for OLED.
Prior art employed various types of seal for OLED. Primarily the methods fall in to two categories namely, perimeter seal and encapsulation seal. In some cases a combination of perimeter and encapsulation seals were employed. Encapsulation method relates to the complete intimate sealing of OLED structure by depositing sealant layers on top of the structure. Perimeter method relates to the sealing of the device at the edges of OLED structure, with its substrate, through the use of a lid. Jones et.al (U.S. Pat. No. 6,198,220) described an encapsulation method that incorporated alternate thin film layers of dielectric and metallic films over the OLED structure. The thickness of each layer was in the range of thousands of angstroms. Jones et. al stated that the pin-holes in the layers are self-sealed by the reaction of moisture around the hole. Before this reaction is complete, the moisture can pass through pin-holes to the sensitive layers of the device and damage the device. Hence, this is not a reliable method. Further, these films deposited through plasma enhanced chemical vapor deposition (PECVD), sputtering and CVD in a multi-layer stack are stressful and develop micro-cracks, in addition to being porous. Further, the UV from the plasma can cause damage to the organic films. Liu et.al described a laser method of solder seal (U.S. Pat. No. 6,608,283) in which a metallic flange containing a glass window was sealed to the substrate containing active matrix OLED structure. The heat developed during this process was so high to incorporate a heat sink to protect OLED from thermal damage. As the laser seal needs to be done fast to prevent the development of heat, the stress introduced during this process can cause damage to the glass window that is pre-sealed to the metallic flange. Further this method is susceptible for ‘bubble-trap’ due to its transient nature. Ghosh et.al described (U.S. Pat. No. 6,706,316) an ultra-sonic method of solder metal perimeter seal to protect the OLED structure. A recessed holder, and a high pressure in the range of 80–100 psi, was necessary to accomplish the seal. The surfaces of the lid and the substrate, employed for the seal, were of dielectric. The molten metal can not establish good bond to these surfaces. Hence this method of hermetic seal is not reliable. Graff et.al described multi-layer barrier coatings, encapsulation method, on OLED claiming the use of almost all types of materials known in the literature and stating that there is no limit on the thickness of these coatings. It is well known in the literature that there is a stress created in multi-layer coatings that leads to micro-cracks in the coatings. Multi-layer coatings on OLED, done so far, never exceeded a few microns. For a good seal, seal thickness should be in substantial fractions of mm. Silvernail et.al described (U.S. Pat. No. 6,614,057) a perimeter seal that has two adjacent layers with no gap between them. The inner layer is claimed to protect the out-gassing from the outer layer during the curing process. However, the inventors employed inner layers out of UV epoxy and thermal epoxy, which is known to out-gass during the curing process and further two adjacent seals of different materials increase the process steps and cost of manufacture.